As described in the above-referenced 015 application, historically the design and testing of radio wave antennas has been principally concerned with antenna gain along its boresight (main beam axis). For this purpose, as shown diagrammatically in FIG. 1, an antenna 10 the performance of which is to be measured may be mounted within an indoor compact test range 12, such as an EMI-shielded anechoic chamber, that is configured to eliminate reflections and interference from unwanted sources of electromagnetic radiation.
Testing of any antenna typically involves directing radio wave emissions from a test signal source 14 toward the antenna, and measuring the antenna's response by a range receiver 16, the output of which may be displayed or recorded via an associated test and measurement workstation 18. Varying the primary axis of the antenna 10 and test signal source 14 (for example, by varying the orientation in orthogonal principal planes of either the antenna or the test source), enables both boresight and off-axis flexibility of performance parameters including gain, polarization, etc., of the antenna to be measured.
Unfortunately, at relatively low frequencies (e.g., UHF), the size of the indoor test range needed to test the antenna becomes physically and cost-wise prohibitive, making it necessary to test the antenna outdoors. While finding an `open air` location to set up an antenna test range that is free of interferers may not have been particularly difficult several decades ago, it has recently become a significant problem, principally as a result of the proliferation of wireless commercial products, such as cellular phones and citizen band radios, as well as specular reflections from buildings and the like. Moreover, not only should the test range be free of interference from outside sources, but it is desired that the test range emissions themselves not interfere with other `off-range` communication equipment. This interference and reflection free test range problem is compounded by the fact that, in addition to measuring main lobe performance, antenna designers are interested in the antenna's off-axis or sidelobe characteristics, that will allow placement of nulls on one or more interferers, such as a cellular radio transmission tower.
Advantageously, the invention described in the 015 application is designed to effectively alleviate this test range impairment problem by employing a spread spectrum signal as the test signal. Because a spread spectrum signal has high autocorrelation properties with itself and high cross-correlation properties with other signals including interferers, as well as time delayed versions of itself due to specular reflection from multipath, it provides a means for enabling only the intended receiver that processes the energy received by the antenna under test to electronically reject all other signals that may be present in the test range, and thereby allows both main beam and sidelobe, off-axis performance of the antenna to be accurately measured, while also preventing interference with other communication equipment.
Now even though spread spectrum signal processing provides an effective means of achieving many dB of processing gain, by spreading out over a wide bandwidth and thereby substantially reducing the influence of energy from unwanted test range interferers, the degree of improvement may be influenced by operational conditions of the test range and circuit parameters of the test range equipment.
For example, as diagrammatically shown in FIG. 2, where the test signal source 14 is positioned at an off-axis location 15 for the purpose of conducting a sidelobe measurement, the presence of a strong interferer 21 in the antenna's main beam 11 (which typically has a substantially larger gain than a sidelobe), may diminish the ability to resolve the sidelobe.
To overcome this problem it is necessary to increase the spreading processing gain--namely substantially increase the chip rate of the spreading sequence of the test signal. While this can be achieved using very high speed electronic components, doing so may add a substantial cost to both the test signal emitter and the receiver processing equipment. A second problem is the fact that reasonably priced RF mixer circuits that are used to modulate the RF carrier with the spreading signal, suffer some degree of leakage of the local oscillator signal (e.g., as a 30 dB down spur). While this carrier spur leakage problem can also be reduced by using more complex mixer circuitry (which usually requires very fine tuning), such circuitry would also add further expense to the test signal generator and receiver processing equipment.